Steven specializes in 17th and 18th century European thought, with a focus on the works of Georg Christoph Lichtenberg and Immanuel Kant. His research addresses foundational questions in epistemology, metaphysics, and philosophical anthropology, with particular attention to idealism and its historical development. Tester has explored Lichtenberg’s empiricist critiques and their connections to Kantian philosophy, offering a nuanced perspective on concepts like self-consciousness and identity within the broader context of Enlightenment thought​​.